1. Field of the Invention
The present invention relates to a coordinate system correcting apparatus for a machine tool.
2. Description of the Related Art
When a machine tool body or a worktable for placing or a workpiece thereon is distorted, a coordinate system comprising orthogonal axes X, Y, and Z set in a work space of the machine tool is also distorted, and thus the positioning accuracy and processing accuracy are adversely affected. To solve this problem, a known coordinate system correcting apparatus (for example, as disclosed in Japanese Unexamined Patent Publication No. 63-37402) divides any one (for example, the axis Z) of the three axes into segments at predetermined intervals. In each of planes orthogonally crossing the axis Z at the respective points of division, the apparatus precisely measures the coordinates on the axes X and Y of a group of predetermined locations, using, for example, an optical measuring apparatus disposed on a different coordinate system. According to a result of the measurement, the apparatus finds distortions from coordinates on normal axes X and Y, and based on the distortions, corrects the coordinates on the axes X and Y in each of the divided segments of the axis Z.
Since this coordinate system correcting apparatus selects one axis as a reference, to measure distortions of the other two axes, and determines correction quantities according to a result of the measurement, a distortion of the reference axis itself will not be corrected. In addition, distortions of the other axes caused by a distortion of the reference axis at an intermediate point between adjacent points of the division will not be corrected. This apparatus, therefore, cannot precisely position and process a workpiece.